Recently, a thin film transistor liquid crystal display panel (TFT LCD) has already become the mainstream product in the market with its outstanding performance. The thin film transistor liquid crystal display panel is mainly composed of an array substrate, a color filter substrate, and a liquid crystal layer. On the array substrate, there are a plurality of thin film transistors arranged in the form an array and pixel units configured to correspond to said thin film transistors. The thin film transistor, as a switch element to actuate the pixel unit's execution, receives a scanning signal from a scan driving circuit via a scanning line, and a data signal from a data driving circuit via a data line, and writes the data signal into the pixel unit under the action of the scanning signal, such that respective liquid crystal molecules of the pixel unit occur corresponding deflection under the influence of the data signal, which causes a certain amount of light to get through.
With respect to the thin film transistor liquid crystal panel of a type of vertical alignment (VA), there is prevalence of the color shift problem. This is because the effective refractive index of the liquid crystal molecules varies with different viewing angles, which thereby causes the change in the intensity of transmitted light. It is specifically manifested in that, light transmission capability reduces under an oblique viewing angle, and color inconsistency is presented in the directions from the oblique viewing angle to the front viewing angle, and especially, a significant color distortion may be observed over a wide viewing angle. Accordingly, one of the important research and development topics of the LCD panel technology is how to achieve a lower color shift.
So far, the mainstream LCD panel manufacturers have generally applied a charge sharing technology to solve these problems. FIG. 1 is a diagram of an equivalent circuit of one unit pixel in the liquid crystal panel display using said charge sharing technique. The pixel electrodes of each pixel unit include two portions, i.e., a main-area electrode and a sub-area electrode, wherein the main-area electrode is driven by a thin film transistor TFT_A; and the sub-area electrode is driven by a thin film transistor TFT_B. The main and sub area electrodes are respectively applied with different voltage levels while they are driven by a voltage of the same gray scale, such that a gray scale curve to be combined with both areas over a wide viewing angle presents a decreased difference from that over the front viewing angle, whereby the color shift problem caused by different viewing angles can be relieved.
Specifically, as the liquid crystal display panel performs the progressive scan driving, when line n is scanned, the voltage level of a scanning signal on the scanning line Gn presents to be high while that of the scanning line Gn+1 is low, and thereby the thin film transistors TFT_A and TFT_B both turn on, and transistors TFT_C1 turns off. As such, under the action of a data signal on the data line Data, a liquid crystal capacitor Clc_A and a storage capacitor Cst_A of the main-area electrode, and a liquid crystal capacitor Clc_B and a storage capacitor Cst_B of the sub-area electrode all proceed to charge until the data signal voltage level is reached, such that both voltages of the main and sub area electrodes reach to a voltage level of the data signal. When line n+1 is scanned, the level of the scanning signal on the scanning line Gn turns to be low while that of the scanning line Gn+1 turns to be high, and the thin film transistors TFT_A and TFT_B thus both turn off while transistors TFT_C1 turns on. As such, the voltage of the sub-area electrode begins to change by means of a charge capacitance Cs1 so as to obtain a certain degree of difference from the voltage level of the main-area electrode. By means of such voltage difference, a lower color shift is accomplished.
In the method described above, the role of the charge capacitance Cs1 is critical, and the magnitude of the capacitance value determines the degree of the voltage difference between the main-area electrode and the sub-area electrode. It is set that ΔV_B is the voltage difference between the main-area electrode and a common electrode, and ΔV_A is the voltage difference between the sub-area electrode and the common electrode, the ratio wherebetween is below:ΔV—B/ΔV—A=(Cst—B+Clc—B)/(Cst—B+Clc—B+2Cs1).
This ratio is a highly crucial parameter in the design, and it depends on the capacitance value of the charge capacitance Cs1. In the practical preparation, the structure of the capacitance Cs1 is generally as shown in FIG. 2a, wherein metal layers M1 and M2 act as both pole plates of the capacitance Cs1, and an SiN insulating layer and an a-Si amorphous silicon semiconductor layer are disposed therebetween. The metal layer M2 is connected to the thin film transistor TFT_C1, and the metal layer M1 is to the common electrode. The C-V characteristic curve of this capacitance is shown in FIG. 2b, which is characterized by that a capacitance value of the positive half cycle (under the positive polarity inversion driving period) is greater than that of the negative half cycle (under the negative polarity inversion driving period). An ideal solution of color shift is that the ratio in the positive polarity inversion driving period (the voltage level of the data signal is greater than that of the common electrode) is consistent with that of the negative inversion driving period (the voltage level in the data signal is lower than the common electrode voltage). However, since the capacitance value of the capacitance Cs1 in the positive half cycle is greater than that in the negative half cycle, the ratio during the positive polarity inversion driving period is thus less than the ratio during the negative inversion driving period, i.e., the ratios during the positive and negative polarity inversion driving periods are different. Such asymmetry may not only degrade the effect of decreased color shift over a wide angle gap, but also bring in, such as, the afterimage phenomenon.